Memento

Context: Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.

public class Memento
{
    public string State { get; }
    public Memento(string state) => State = state;
}

public class Originator
{
    public string State { get; set; }
    public Memento Save() => new Memento(State);
    public void Restore(Memento m) => State = m.State;
}

public class Caretaker
{
    public Memento Memento { get; set; }
}

Real-world usage example

Undo mechanism: A text editor saves a memento before each change. The caretaker (history stack) stores mementos to restore previous states.

Example: In C#, serialization (binary, XML, JSON) can be used to save object state – a form of memento. Also, ITrackable in Entity Framework Core tracks original values for concurrency.

Related sections

• 5.3 Behavioural patterns
• 5.3.5 Mediator
• 5.3.7 State